Oil has improved a lot since the old Farmalls were built, but the functions of oil remain the same: cool, clean, lubricate, and seal. Modern multi-grade oils will do everything your old M requires, and do it far better than the SAE 30 non-detergent stuff that was in it when it left the factory.
Starting an engine when it's very cold, or after it has set idle for a long time are two of the highest-wear situations it will experience. The thinner (at low temperature) SAE 10W30 will allow it to get vital lubrication sooner than straight SAE 30 when the temperatures are very low. If the engine is in good enough shape to maintain proper oil pressure when it warms up, 10W30 won't hurt it a bit! If the engine is nearing the time of a needed overhaul, put in 10W40, and the oil pressure will stay up a little better.
Many of the old Farmalls don't work hard enough any more, or long enough at one time, to get the oil as hot as it used to get when the tractor was out in the field tilling the ground all day. For this reason, modern high-detergent oils will keep the inside of it cleaner, and better fight the effects of condensation in an engine that runs for brief amounts of time.
I've run 10W30 in my M and SM for years in the winter, and have pushed lots of snow with each. They've each run lots of hours doing tillage and planting with either 10W30 or 10W40 in them. Oil consumption has not been a problem, and neither has suffered any damage.